Dimethyl carbonate is useful as a starting material in the synthesis of aromatic polycarbonates, as a solvent, and as a reactant in the production of a number of chemical products. Dimethyl carbonate is often present in a mixture with methanol. In various processes to produce dimethyl carbonate, for instance, dimethyl carbonate is obtained in a mixture with methanol. Dimethyl carbonate and methanol form an azeotropic composition having a weight ratio of dimethyl carbonate to methanol of about 30:70 at atmospheric pressure.
It is necessary, therefore, to separate the dimethyl carbonate from the dimethyl carbonate/methanol mixture. Processes used to separate the azeotropic mixture of dimethyl carbonate and methanol include pressure swing distillation, high pressure distillation and extractive distillation.
Extractive distillation is a method to separate components of a mixture that exhibits azeotropic behavior. In an extractive distillation process, an agent, herein referred to as an "extractive distillation agent" is added to mixture of components to be separated in the distillation process, so that the relative volatilities of the components of the mixture are changed. The change in relative volatilities is such that a sufficient difference in volatility of the components results, and effective separation by distillation becomes possible.
U.S. Pat. No. 5,292,917 discloses a process for the extractive distillation of a dimethyl carbonate/methanol mixture by distilling the mixture in the presence of dimethyl oxalate. Two distillation columns are used in series to separate the dimethyl carbonate from the methanol. In the first column, methanol is removed at the top of the column, and the dimethyl carbonate/dimethyl oxalate mixture is fed to the second column. In the second column, the dimethyl carbonate is separated from the dimethyl oxalate.
Japanese Publication 06228026, published on Aug. 16, 1994, discloses a method of extractive distillation of a mixture of methyl carbonate and methanol in the presence of an extractive agent selected from a group of compounds, including anisole.
Japanese Publication 06016596, published on Jan. 25, 1994, discloses a method of separating dimethyl carbonate from methanol by refluxing the dimethyl carbonate/methanol mixture in the presence of an organic solvent, e.g. phenol, and removing methanol as an overhead product and a mixture of the organic solvent and dimethyl carbonate as the bottom product.
Known methods for separating a dimethyl carbonate/methanol mixture involve at least two distillation columns. Therefore, there exists a need for a simplified and more economical process for separating a dimethyl carbonate/methanol mixture.